  <tr class="detalle_guia">
    <% fields_for_associated :guia, detalle_guia do |v|%>
      <td><%= v.collection_select(:centro_id, @centros, :id, :nombre_comercial, {:include_blank => true}, :index => nil) %></td>   
      <td> <%= select_tag "guia[detalle_guias_attributes]__mes_factura", options_for_select(@meses), :name => 'guia[detalle_guias_attributes][][mes_factura]'%> </td>
      <td><%= v.text_field(:entregadas, :size => '6', :index => nil) %></td>
      <td><%= delete_link_for(detalle_guia, "Quitar", v) %></td>
    <% end %>
         
       </tr>